As one of the methods for removing impurities in contaminated water, known is a chemical method in which some chemicals are added to the contaminated water as flocculants to coagulate the impurities contained in the contaminated water using a counter ion effect. Particles having a dimension ranging from 10−9 to 10−6 m, which are suspended in the contaminated water, are called colloids. As colloids cannot be easily separated by natural precipitation or filtration, the above chemical method is intended to coagulate such fine particles to form larger particles to a certain extent and separate them.
Colloid particles have been charged, and positive ions are attached onto the surface of a negatively charged particle to cause specific absorption, around which a diffusion layer of ions is formed. The movement of the particles generates zeta potential of −20 to −30 mV on a slip plane between the particle and the diffusion layer. As the zeta potential generates a major portion of the repulsion energy between the particles to form an energy barrier, a flocculant is used to neutralize the zeta potential, thereby reducing the potential barrier to make the particles absorb each other.
Flocculants are roughly divided into inorganic salts, organic high-molecular compounds and flocculation agents. In the inorganic salts which are often used, aluminum sulfate, basic aluminum chloride, ferric sulfate and ferric chloride are exemplified as typical ones. When a flocculant of an inorganic salt is added to contaminated water and stirred, positively charged metal ions in the flocculant neutralize the negative zeta potential of the colloid particles in the contaminated water and absorb them.
Thus, when a flocculant is added for neutralizing the zeta potential of colloid particles, positively charged metal ions are required in an amount corresponding to the negative zeta potential. Therefore, the larger the negative zeta potential in the contaminated water, the more metal ions required to be reacted, and it becomes necessary to add a large amount of flocculant for removing the impurities in the contaminated water.
If a large amount of flocculant is added for coagulation, the amount of the coagulated sediments also becomes larger depending on the increased flocculant, which leads to an additional burden in the work for collecting the coagulated sediments. Furthermore, some kinds of flocculants would affect nature. Accordingly, it is desirable to control the amount of flocculant used as small as possible.
In view of the above, the object of the present invention is to provide an apparatus and a method for purification of water which enable the efficient removal of impurities in contaminated water with a reduced amount of flocculant.